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Version: 4.0 (Archived) 📦

TimeJitter

描述了SRS的时间戳矫正机制。

RTMP Monotonically Increase Timestamp

RTMP要求流的时间戳是单增的,视频流的时间戳单增,音频流的时间戳单增。所谓单增就是指单调递增,所谓单调递增就是包的时间戳越来越大。所谓越来越大就是......数字越来越大。

单增有两种情况:

  • 分别单增:指的是视频是单增的,音频是单增的,但是流(混合了音频和视频)是不单增的。
  • 流单增:指的不仅仅是分别单增,而且流里面的包永远是单增的。

RTMP协议没有说道要求什么级别的单增,但一般指流单增。

如果非单增会怎样?有些服务器会断开连接,librtmp会报错,flash客户端会播放不了之类。但是,实际上并没有那么恐怖(还是保持单增好点,毕竟RTMP协议里说到这个了),所以有些编码器出来的流不是单增也能播放,特别是用vlc播放之类。

Timestamp Jitter

如果流不是单增的怎么办?SRS采用非常简单的算法保证它是单增的。如果不是单增就把时间戳增量设为40(即fps为25)。这个机制就是SRS的时间戳矫正机制。

有几处地方用到了时间戳矫正:

  • RTMP流分发:可以设置vhost的time_jitter来选择矫正机制。分发给客户端的RTMP流的时间戳矫正机制。
  • DVR录制:可以设置vhost的dvr的time_jitter来配置矫正机制。录制为flv文件的时间戳处理机制。
  • HLS:打开时间戳矫正机制。
  • Forward:打开时间戳矫正机制。
  • HTTP Audio Stream Fast Cache: 和RTMP一样,即在vhost中配置,参考fast_cache.

如果你的编码器只能做到分别单增(对音频和视频分别编码的情况很常见),那么可以关闭时间戳矫正。

Config

在vhost中配置时间戳矫正:

vhost jitter.srs.com {
# for play client, both RTMP and other stream clients,
# for instance, the HTTP FLV stream clients.
play {
# about the stream monotonically increasing:
# 1. video timestamp is monotonically increasing,
# 2. audio timestamp is monotonically increasing,
# 3. video and audio timestamp is interleaved/mixed monotonically increasing.
# it's specified by RTMP specification, @see 3. Byte Order, Alignment, and Time Format
# however, some encoder cannot provides this feature, please set this to off to ignore time jitter.
# the time jitter algorithm:
# 1. full, to ensure stream start at zero, and ensure stream monotonically increasing.
# 2. zero, only ensure sttream start at zero, ignore timestamp jitter.
# 3. off, disable the time jitter algorithm, like atc.
# default: full
time_jitter full;
# whether use the interleaved/mixed algorithm to correct the timestamp.
# if on, always ensure the timestamp of audio+video is interleaved/mixed monotonically increase.
# if off, use time_jitter to correct the timestamp if required.
# default: off
mix_correct off;
}
}

其中,vhost的mix_correct配置,能将分别单增的音频和视频流,变成混合单增的流。

在DVR中配置时间戳矫正:

vhost dvr.srs.com {
# dvr RTMP stream to file,
# start to record to file when encoder publish,
# reap flv according by specified dvr_plan.
# http callbacks:
# @see http callback on_dvr_hss_reap_flv on http_hooks section.
dvr {
# about the stream monotonically increasing:
# 1. video timestamp is monotonically increasing,
# 2. audio timestamp is monotonically increasing,
# 3. video and audio timestamp is interleaved monotonically increasing.
# it's specified by RTMP specification, @see 3. Byte Order, Alignment, and Time Format
# however, some encoder cannot provides this feature, please set this to off to ignore time jitter.
# the time jitter algorithm:
# 1. full, to ensure stream start at zero, and ensure stream monotonically increasing.
# 2. zero, only ensure sttream start at zero, ignore timestamp jitter.
# 3. off, disable the time jitter algorithm, like atc.
# default: full
time_jitter full;
}
}

ATC

RTMP ATC开启时,RTMP流分发的时间戳矫正机制变为关闭,不对时间戳做任何处理。

Winlin 2015.4